The invention relates generally to a system and a method for electrochemical energy conversion and storage and more specifically to materials, methods and apparatus of electrochemical energy conversion and storage using an organic liquid carrier of hydrogen.
Many electrochemical energy conversion and storage devices such as secondary batteries (e.g. lithium-ion batteries, nickel metal hydride batteries, sodium metal chloride batteries, etc.), electrochemical supercapacitors, and fuel cells are known. It is noted that the electrochemical supercapacitors have low energy density, while batteries are expensive and are not suitable for mobile and stationary applications.
Proton exchange membrane (PEM) based fuel cells are considered to be effective electricity generators for both stationary and mobile applications. PEM fuel cells electrochemically react air with an external supply of fuel to produce electricity and typically have an energy density that is greater than conventional electrochemical batteries. Typical fuel for a PEM fuel cell is hydrogen. Effective hydrogen storage remains a challenge, especially for mobile applications. High pressure or liquid hydrogen storage options are too expensive and typically have a low volumetric energy density. Current solid materials for hydrogen storage operating at temperatures below the typical operating temperatures of PEM fuel cells (100° C.) are currently capable of storing only about 4 weight percent and require a sophisticated heat management system that reduces total system capacity by about 50 percent or more. In addition such materials require total redesign of cars and refueling infrastructure. Liquid fuels like methanol also can be used in PEM fuel cells. However, these fuels generate CO2 and CO that poisons the fuel cell catalyst. The most effective type of fuel for a PEM fuel cell is methanol that is a very toxic and highly flammable liquid. The use of a diluted methanol fuel reduces these risks but also substantially reduces the system energy density.
To improve the energy density of the PEM fuel cell system, many efforts are focused on improvement of the hydrogen storage subsystem. Some high capacity metal hydride options currently exist but they are either irreversible or work reversibly at much higher temperatures than the fuel cell operates. The recharge of these hydrides involves a high rate of heat dissipation and therefore additional components such as a heat exchanger.
Accordingly, there is a need in the art for an improved electrochemical energy conversion and storage system that overcomes some of the limitations of the current PEM fuel cells and hydrogen storage limitations.